An algorithm for calculating the chemical potential in associating and reacting fluids

We present a Monte Carlo algorithm to determine the chemical potential of associating and reacting fluids. The algorithm is based on the fact that the chemical potential of a component is the same in the monomer (unbonded) state as in any bonded state. We demonstrate that the chemical potential of the unbonded specie can be calculated at relatively low cost and with high accuracy. The algorithm is applicable to both homogeneous as well as inhomogeneous systems. We compare the results of the presented algorithm against the findings of Widom's single stage particle insertion method for four commonly encountered inhomogeneous systems of associating fluids in phase equilibria studies. The constancy of the chemical potential throughout an inhomogeneous system under equilibrium is used as an independent test of the algorithm. The uncertainty in chemical potential values over the system for the cases studied was on an average 30 times smaller in the new algorithm, with at least 5 times fewer insertions than in the traditional Widom's method.